Abstract : A satellite-borne detector for protons and alpha particles has been developed and flown. The detector uses two totally depleted silicon surface barrier detectors in a coincidence telescope arrangement to detect 5-100 MeV protons and 20-100 MeV protons and 20-100 MeV alpha particles. A calibration mode allows the front detector to be used to detect 1-5 MeV protons and 10-30 MeV alpha particles. The report contains a detailed theoretical treatment of the telescope operation, including the rejection capability for electron detection, and a description of the electronics. Results from a telescope orbiting in satellite OV5-6 are presented. Calibration mode data from quiet times show the system electronic gains to be substantially the same as before launch while data from outer trapped electron belt show the telescope to be highly immune to electron counting. (Author Modified Abstract)